258 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



ventral mesentery. In the ventral part of the mesentery below 

 the fore-gut is swung the heart ; the mesocardia disappear ; but 

 that part of the mesentery behind the heart, in which the sinus 

 venosus rests and in which the stomach is developed, persists 

 (Fig. 201.) The ventral and dorsal parts of the mesentery of 

 this region form the first or mesial element of the diaphragm. In 

 this part of the mesentery lie (Figs. 211 and 212) (1) the 

 stomach, (2) the posterior end of the sinus venosus ; (3) the 

 vitelline and umbilical veins which pass in it to the sinus; 

 (4) the ducts of Cuvier which enter it to reach the sinus ; (5) the 

 liver bud which is developed into it ; (6) the ductus venosus 

 and pre-renal part of the inferior vena cava which are de- 

 veloped in it. Further, on each side of this part of the mesentery 

 lies the isthmus of the coelom into which the lung buds are 

 developed and out of which the pleural cavities are formed 

 (Fig. 205). The mesial element is situated, at first, under the 

 middle cervical segments of the trunk. 



In the course of development the stomach gradually separates 

 itself from this part of the diaphragm ; the liver bud also frees 

 itself ; the ducts of Cuvier, with the sinus venosus, also become 

 separated from it. 



2. The formation of the ventro-lateral parts of the diaphragm 

 is obscure. They appear very early (3rd week), while yet the 

 vitelline veins terminate in the sinus venosus, and before the liver 

 bud has grown out (Fig. 187, p. 229). It will be seen from 

 Figs. 187 and 212 that the vitelline veins pass obliquely in the 

 ventral mesentery from the yolk sac to the sinus venosus. They 

 are said to be the active agents in producing the ventro-lateral 

 parts of the diaphragm. They become so dilated (see Fig. 211) 

 as to produce their covering of splanchnopleure outwards until 

 lateral folds are formed which come in contact with, and adhere 

 to, the somatopleure. In this way the mesentery (mesial part 

 of the diaphragm) becomes adherent to the lateral somatic walls. 

 These three parts, mesial and ventro-lateral, form the septum 

 transversum. 



3. The Dorso-lateral Parts. — These parts are formed in the 

 3rd month, and arise as cresentic folds from the dorsum of the 

 isthmus. They are situated at first under the cervical region 

 and their free cresentic edges are directed backwards over the 



